RFID technologies are widely used for automatic identification. A basic RFID system includes an RFID tag or transponder carrying identification data and an RFID interrogator or reader that reads and/or writes the identification data. An RFID tag typically includes a microchip for data storage and processing, and a coupling element, such as an antenna coil, for communication. Tags may be classified as active or passive. Active tags have built-in power sources while passive tags are powered by radio waves received from the reader and thus cannot initiate any communications.
An RFID reader operates by writing data into the tags or interrogating tags for their data through a radio-frequency (RF) interface. During interrogation, the reader forms and transmits RF waves, which are used by tags to generate response data according to information stored therein. The reader also detects reflected or backscattered signals from the tags at the same frequency, or, in the case of a chirped interrogation waveform, at a slightly different frequency.
The reader may detect the reflected or backscattered signal by mixing this signal with a local oscillator signal. This detection mechanism is known as homodyne architecture. Separate antennas for transmission and reception can be used, as discussed in, for example, U.S. Pat. No. 6,114,971. In this case effective isolation of the received signal from the transmitter can be easily obtained, but two antennas separated by a significant distance or a shielding structure of some kind must be used. Efficient antennas are physically large, particularly at low frequencies such as c. 450 MHz or c. 900 MHz, and the use of two antennas necessitates a large interrogating device, which is incompatible with the portability and versatility features desirable in many RFID applications.
RFID readers using only one antenna are also known in the art, as described, for example, in U.S. Pat. No. 6,107,910. RFID readers with a single antenna for both transmit and receive functions are developed by employing a microwave circulator or directional coupler to separate the received signal from the transmitted signal. A tapped transmission line serves as both a phase shifter and directional coupler alternatively may be used, such as the one described in U.S. Pat. No. 5,850,187. Such devices, however, are not capable of distinguishing between the modulated reflected signal from the RFID tag, placed typically at a distance of some meters from the antenna, and the reflection of the transmitted signal from the antenna structure itself. The reflection is typically caused by the inevitably imperfect match of the antenna input impedance to the transmission line impedance of the antenna feed connection.
The reflection of the transmitted signal can present significant impediment to achieving good sensitivity in the detection of backscattered signal the passive RFID tag. This can be explained further using the example illustrated in FIG. 1, which shows an RF transmitter 110 connected to an antenna 120 via a coupler or isolator 130. The transmitter 110 is shown to comprise a microprocessor system controller 112, a frequency synthesizer 114, an optional modulator 116, and an amplifier 118. An RF receiver (not shown) is also coupled to the antenna 120 through the coupler 130. If the transmitter has a 1 watt or 30 dBm output power at 925 MHz and the antenna, being very well matched, has a return loss of −15 dB, the reflected transmit power captured by the coupler or isolator will be (30−15)=15 dBm, ignoring incidental losses in the circuitry. The coupler removes only a portion of the reflected signal, for example, 10% of the reflected power for a 10 dB coupler. So, the resulting reflected power into the receiver will be approximately 5 dBm. Such a large signal creates significant obstacles in the attempt to detect the tiny backscattered signal from the RFID tag, which may be as small as −90 to −100 dBm.
DC offsets due to second-order distortion of the large reflected signal in the receive chain will occur in a homodyne radio, in which the LO and received/reflected signals are at the same frequency. AC coupling of the amplifier chain may be prohibited due to loss of information near zero frequency, depending on the protocol; in this case, the DC offsets will be multiplied by the gain of the chain, which may be as high as 100 dB in order to detect small backscattered signals, leading to saturation of the downstream amplifiers and desensitization to the wanted signal. Even in the case where DC signals are not of consequence, as for example when the RFID tag is modulated at a subcarrier frequency of a few MHz, the large reflected signal is present in all the amplifier stages prior to channel filtering, and can lead to saturation, desensitization, and recovery problems in the transition from the modulated transmitted signal to the continuous wave (CW) receive state which is typically employed in passive tag interrogation. For all these reasons it would be desirable to remove as much of the reflected signal as possible from the receiver input.
A perfectly matched antenna structure cannot be guaranteed by design or manufacture, due to the variability in the near-antenna environment, which is encountered during actual use of the reader. An adaptive antenna tuning circuit, combined with appropriate optimization/calibration algorithms, could be considered for this purpose. However, mechanical tuners cannot respond on the sub-millisecond time scale required in frequency-hopping systems typically used for RFID applications in unlicensed bands. Electrical tuning elements have sufficiently fast response times to retune rapidly after a frequency hop, but are typically constructed using, as their variable elements, voltage-sensitive devices such as voltage-variable capacitors (varactors), which change their impedance in a complex fashion when exposed to the large RF transmit voltages, giving rise to unacceptable transmit distortion, degraded return loss, and spurious radiated output frequencies.
What is needed is an adaptive method of removing all or most of the antenna reflected signal from the receiver signal.